Polyvinyl chloride (PVC) is the best known and most widely used vinyl polymer. It is used mainly in two forms: (i) rigid PVC; and (ii) flexible PVC. The rigid form of PVC, known as non-plasticized PVC, is generally used to produce pipes, connections, profiles and frames, as well as applications where chemical resistance is necessary. The flexible form of PVC, also known as plasticized PVC, is used in films, blankets, electrical insulators, flooring, toys, wallpapers, synthetic leather used for clothes, footwear and coatings, among many other end products.
Malleability and flexibility are physical characteristics that may be modulated by formulating a specific polymeric resin with one or more materials that can serve as plasticizers. In a broad definition, a plasticizer may be understood as a substance with a high boiling point that when included in a polymeric matrix confers and preserves the flexibility of the material. The plasticizer become an integral part of the polymer and must provide the benefits of plastification throughout the entire useful life of the product.
One of the most relevant characteristics of a plasticizer is its compatibility with the polymeric resin. The primary plasticizers are highly compatible with the polymer and may be included to it in large amounts, with no exudation. In addition to the above-mentioned malleability and flexibility, these additives may provide better coloring, easier processing and broader range of applications. The primary PVC plasticizers are generally petroleum derivatives, among which are phthalates, adipates, trimellitates, benzoates, azelates and polymers.
The secondary plasticizers are moderately compatible with the polymeric resin and are generally used together with a primary plasticizer in order to reduce costs or to obtain specific properties. The chlorinated paraffins (PCs) are the most common examples of secondary plasticizers for PVC resins, widely used as they present low flammability and low cost. Other examples of secondary plasticizers are the poly-alpha methyl styrene derivatives and vegetable oils derivatives.
Some concerns regarding the toxicity and performance of some primary plasticizers traditionally used by the halogenated plastics industry, and the constant need to improve PVC formulations, have driven the search for alternatives. In the recent years, much interest has been focused on plasticizers that may be obtained from precursors generated through microbiological processes or from renewable sources, such as citrates and modified vegetable oils.
For example, epoxidized vegetable oils have been used in small proportions as secondary plasticizers and thermo-costabilizers in the production of flexible, semi-rigid and rigid PVCs. However, the use of epoxidized triglycerides as primary plasticizers results in exudation due to the limited compatibility with the polymeric matrix. In order to enhance the compatibility of the vegetable oils with PVC, some modifications, in addition to epoxidation, were proposed: esterified (GB 1020866), interesterified and acetylated derivatives were suggested as primary plasticizers for PVC resins. Among the products generated by these processes are the acetylated/epoxidized mono- and di-glycerides and epoxidized fatty acid alkyl esters.
The methods for production of epoxidized fatty acid alkyl esters generally involve two steps. The first step is to transform fatty acids or a vegetable oil into fatty acid alkyl esters. This transformation may be conducted through esterification (when the precursors are fatty acids) or transesterification (when the precursors are vegetable oils or fatty esters with short alkyl chain).
The esterification is a reaction between carboxylic acids and alcohols that generates esters and water as products. As this is a balance reaction, the water produced is generally removed from the system in order to favor the formation of the ester. Fatty acids esterification is normally conducted in the presence of an acid catalyst (for example, H3PO4, H2SO4, CH3C6H4SO3H, CH3SO3H, among others), at temperatures higher than 120° C. Titanates may also be used as catalysts in esterification, however higher temperatures are required (>200° C.) in order to the reaction be effective. When alcohols with high boiling points are used, temperatures of over 230° C. allow esterification to be achieved with no catalyst, however, the end product is dark and requires treatment to improve its coloring.
The transesterification is a process through which an ester is reacted with an alcohol in order to form a new ester and an alcohol resulted from the initial ester. This reaction may be used when the precursors are low acidity vegetable oils or fatty acid esters with short alkyl chain—typically methyl or ethyl esters. The transesterification is normally conducted at moderate temperatures (below 130° C.), requires anhydrous conditions and may be catalyzed by an acid or a base, generally in the homogenous phase. The acid catalyst may be H2SO4, HCl, H3PO4, CH3C6H4SO3H, among others. The basic catalysts most commonly used for transesterification are alkaline metals hydroxides or alcoxides, such as NaOH, KOH, LiOH, NaOCH3 and KOCH2CH3.
The other required step to obtain epoxidized fatty acid alkyl esters is epoxidation. In the epoxidation the double bounds presented in the different fatty acid alkyl ester chains (products obtained through esterification or transesterification) are converted into epoxide groups (or oxirane). This reaction must take place after an esterification step, or may be conducted before or after a base-catalyzed transesterification step.
The epoxide groups may be incorporated by using any appropriate technique. The most widely used procedure is the reaction with a percarboxylic acid, pre-formed or generated in situ through hydrogen peroxide and an aliphatic organic acid, usually formic acid or acetic acid. Said epoxidation techniques are well known in the science. In addition to making the products compatible with the polymeric resin, the presence of oxirane rings in the fatty acid esters chains significantly contributes to the photo-thermal stability of the end material.
As mentioned above, the esterification of carboxylic acids is usually catalyzed by acids and the transesterification may be catalyzed by bases or by acids. For example, BR 0602925-6 describes a process for the preparation of fatty acid esters and their subsequent epoxidation in order to produce plasticizers and the resulting product. The invention describes the preparation of methyl or ethyl esters through transesterification catalyzed by methoxide or ethoxide of an alkaline metal. These esters with short alkyl chain were used as precursors in order to produce other fatty acid alkyl esters from polyols and alcohols with medium and long chains, in transesterification reactions catalyzed by acids, preferably methane sulfonic acid, with the catalyst not varying as a function of the alcohol used. For alcohols with high boiling points, titanates were the preferred catalysts. The obtained esters were subsequently epoxidized with peracid.
In cases where the catalyst used to obtain the alkyl esters is an acid, normally it is not possible to directly convert the raw materials containing epoxide groups, as is the case of epoxidized fatty acids or epoxidized vegetable oils. In the presence of an alcohol and an acid catalyst, the epoxide groups present in these precursors are partially or fully converted into hydroxy ethers. Bearing in mind that the presence of the epoxide group is fundamental for the good performance of the plasticizer, the product presents a reduction in its functionality. In these cases, in order to offset this drawback, the epoxidation stage must always be conducted after the esterification step (or transesterification catalyzed by an acid).
The epoxidized vegetable oils, as well as epoxidized fatty acid methyl or ethyl esters, may be directly converted into epoxidized alkyl esters by base-catalyzed transesterification. In this case, there is not the drawback of parallel reactions between the epoxide groups and the alcohol used in the reaction. However, if the base catalyst is a hydroxide (LiOH, NaOH, KOH, etc.), its reaction with the alcohol forms water. Even if present in small quantities in the reaction mixture, the water results in the formation of soap, occurring reduction in the alkalinity of the catalyst and greater difficulty during the subsequent purification step, due to formation of emulsions. Another problem related to the hydroxides is that when alcohols with medium or long chains are used in transesterification catalyzed by these bases, a good conversion is achieved only with two or more transesterification stages, required to ensure satisfactory conversion of the vegetable oil into fatty acid alkyl esters.
On the other hand, alkaline metal alcoxides, such as sodium methoxide or ethoxide, are effective catalysts and do not form water in the reaction mixture during vegetable oils transesterification. Generally, alcoxides are commercially available in the form of solutions; for example, sodium methoxide is sold as a 30% in methanol solution. For transesterification involving alcohols with medium and long chain, the use of these catalysts in an alcohol solution results in an end product with appreciable quantities of fatty esters with short alkyl chain in its composition.
Epoxidized fatty acids methyl or ethyl esters may satisfactorily serve as PVC plasticizers in less sensitive applications, such as the production of certain calandered and extruded materials. However, the use of these epoxidized fats esters with short alkyl chain or plasticizers containing these compounds may cause problems in some situations. The relatively low molar mass of these compounds may restrict their application, especially when the PVC compounds and plastisols are processed at temperatures above 190° C. Under such conditions, intensive volatization during the processing and exudation in the obtained polymeric materials are noted.
The use of pure alcoxides is one way of reducing the formation of esters with short chain in transesterification reactions that involve alcohols with medium or long chain. However, as a solid, sodium methoxide is extremely poisonous, causing damage to mucous tissues and to the respiratory tract, it is thus avoided as it is difficult to handle. Sodium ethoxide, another alcoxide available in solution, is extremely hygroscopic in its solid form. On the other hand, potassium tert-butoxide is an alcoxide sold as a pure solid and with good stability, nevertheless its price is relatively high making a significant contribution to large scale process costs, as it is the case of PVC plasticizers production.
In their elementary form, alkaline metals are highly reactive and require special care when being handled and used. They react with water, forming hydrogen and hydroxide, and there is the possibility of explosion, depending on the quantity involved. Although knowing that alkaline metals may be used in transesterification processes, the teachings of the state of the art do not suggest said use for producing PVC plasticizers from vegetable oils. This possibly occurs due to the difficulties and risks involved in the use of alkaline metals in their elementary forms, in addition to a false impression of high costs.
The patent application US 20090149586 describes a process for the synthesis of primary plasticizers for PVC comprised by epoxidized fatty acids ethyl or isoamyl esters and PVC compositions prepared with said esters. The catalyst used in the process was sodium hydroxide (3.8% in relation to the soya oil mass) using molar ratios of refined soya oil to alcohol of 1:10 to 1:30, which are far higher than the traditional ratio of 1:6 used in the transesterification of refined oils. After transesterification, the obtained alkyl esters were epoxidized with percarboxylic acid generated in situ. This document neither describes nor suggests the use of alkaline metal as a catalyst precursor. Moreover, attempts to reproduce the teachings set forth in this document indicated the need for at least two transesterification stages in order to obtain an end product with acceptable purity.
There are also other drawbacks in processes for modifying vegetable oils known in the state of the art, which are related to the fatty acid esters purification. When the process involves the transesterification of a vegetable oil, glycerin separation may require prolonged periods, the washing of the product may demand large amounts of water, and the removal of excess alcohol, if it presents a high boiling point, normally requires high vacuum, which requires heavy investments.
It is an objective of the present invention the use of alcohols with medium and long chain in transesterification and obtaining epoxidized fatty acid esters with long and medium alkyl chains, free from epoxidized methyl and ethyl esters.
It is also an objective of the present invention the use of alkaline metal to produce an alcohol derivative alcoxide catalyst used as a transesterification reagent.
Another objective of the present invention is to provide a new and improved process for modifying vegetable oil in order to obtain products that serve as high grade primary plasticizers in vinyl polymer formulations that may be used in high concentrations, providing to the polymer the desired qualitative aspects, maintaining the fundamental qualities of the end material.
Further characteristics, aspects and advantages of the present invention will become more clearly apparent through reading the following descriptions.